Xanthan gum is an acidic exopolysaccharide (EPS) normally secreted by X. campestris (Jeanes, A., et al., 1961, J Appl Polymer Sci 5: 519-526), and is useful as an aqueous rheological control agent because it exhibits high viscosity at low concentration, pseudoplasticity, and insensitivity to a wide range of temperature, pH, and electrolyte conditions (see U.S. Pat. Nos. 5,194,386, 5,472,870, 5,279,961, 5,338,841, and 5,340,743, the contents of each of which are incorporated herein by reference). The genes that code for its synthesis are gumB through M(Capage, M. A., et al., 1987, WO87/05938; Vanderslice, R. W., et al., 1989, the contents of which are incorporated by reference; Genetic engineering of polysaccharide structure in Xanthomonas campestris. In: Biomedical and biotechnological advances in industrial polysaccharides, V. Crescenzi, I. C. M. Dea, S. Paoletti, S. S. Stivala, and I. W. Sutherland, eds, pp 145-156, Gordon and Breach Science Publishers, New York).
A different source of commercially significant and functionally diverse biopolymers is the genus Sphingomonas (Pollock, T. J., 1993, J Gen Microbiol 139: 1939-1945). Different organisms of this genus secrete a variety of different strain-specific exopolysaccharides For example, one species secretes Gellan.RTM., while others secrete welan, rhamsan, S-88 or other polysaccharides (Moorhouse, R., 1987, Structure/property relationships of a family of microbial polysaccharides. In: Industrial polysaccharides: genetic engineering, structure/property relations and applications. M Yalpani, ed, pp 187-206, Elsevier Science Publishers B.V. Amsterdam).
We refer to this group of polymers as "sphingans," after the common genus, because they also have common carbohydrate backbone structures (-x-glucose-glucuronic acid-glucose-; where x is either L-rhamnose or L-mannose) with distinct side chains. (See U.S. patent application Ser. Nos. 08/592,874, filed Jan. 24, 1996, and 08/377,440, filed Jan. 24, 1995, the contents of each of which are hereby incorporated by reference). The structure for sphingan S-88 is shown in FIG. 1. The organization and DNA sequence of 23 genes (FIG. 2) that direct the synthesis of sphingan S-88 have been described (Yamazaki, M, et al., 1996, J Bacteriol 178: 2676-2687).
The commercial production of highly viscous xanthan gum and other bacterial polysaccharides is a complex biosynthetic and process-engineering problem (Kennedy, J. F. et al., 1984, Prog Industrial Microbiol 19: 319-371). The sugar substrate is important primarily because the sugar affects productivity, but the cost of the sugar can also be significant. Currently, xanthan gum is produced by supplying X campestris with corn syrup, sucrose or starch. Yet, three to four typical cheese factories can provide enough low-value lactose-containing waste whey to satisfy all of the existing worldwide demand for xanthan production.
A recombinant strain that can stably convert lactose into xanthan gum in amounts equal to the conversion of glucose is disclosed in U.S. Pat. Nos. 5,434,078, and 5,279,961, the contents of each of which are incorporated herein by reference.
It is desired to improve the methods of the production of xanthan gum to achieve more cost-effectiveness, convenience, more desired product qualities and greater production efficiency.
A problem encountered with xanthan gum produced by conventional methods, is that it is contaminated with a cellulase which can be very disadvantageous in commercial applications where xanthan is mixed with or contacts cellulosic polymers. The result is deterioration of the cellulosic polymers.
Methods are known for the treatment of xanthan gum which has been separated from fermentation broths to remove the cellulase contaminant. However, these treatments require processing of the xanthan gum and add to the expense and overall complexity of the process.